p-Cycle survivable networks were introduced in the late 1990s, with the aim of bringing the simplicity and rapidity of survivable rings in the much more flexible and efficient full mesh-based context. There has been an increased interest in p-cycles over the last decade. But related prior research on p-cycles mainly assumes an optical switching based on the wavelength level of granularity. p-Cycles' configuration on a per-wavelength basis is generally more than attractive from the perspective of flexibility in the routing of working paths and freedom in the selection of protecting structures.
But wavelength switching may also involve higher costs mainly due to operations in the electronic domain. This is often the case in opaque transport networks where wavelength discontinuities require optic-electronic-optic (o-e-o) conversion capabilities at every node across working paths and p-cycles. Wavelength conversion is also partially required, at least at the entry points of working paths into p-cycles, for hybrid p-cycle architectures. In hybrids, a given p-cycle is not actually required to ride onto the same wavelength as the working paths it handles, although working paths and p-cycles are independently assumed transparent, i.e. they must thus keep the same wavelength en route. Actual attempts to eliminate or reduce much of the higher costs due to electronics give rise to full transparent network architectures. Working paths and p-cycles are still transparent but unlike with hybrids, the wavelength used by a given working path must also match the same optical frequency (or “color”) assigned to its protecting cycles. Readers may want to look at D. A. Schupke, M. C. Sheffel and W. D. Grover, “Configuration of p-Cycles in WDM Networks with Partial Wavelength Conversion,” Photonic Network Communications, Kluwer Academy Publishers, vol. 6, no. 3, pp. 239-252, November 2003 for more details of p-cycles' configuration in opaque, hybrid and transparent optical domains.
Glass switched p-cycles constitute a promising alternative to wavelength switched p-cycles in a fully transparent context. The idea is to keep the flexible normal state routing of wavelength based configurations; but use in place of optical cross-connects (OXCs), other cross-connect devices having the ability to switch (at once) all wavelengths of an entire failed fiber optic into a p-cycle formed out of span fibers. In A. Grue, W. D. Grover, M. Clouqueur, D. A. Schupke, D. Baloukov, D. P. Onguetou, B. Forst, Capex Costs of Lightly Loaded Restorable Networks Under a Consistent WDM Layer Cost Model, in: Proceedings of the IEEE International Conference on Communications (ICC), Dresden, Germany, 2009, such a fiber-level protection was proven to have a great potential for eliminating wavelength continuity constraint and computational complexity issues in fully transparent p-cycle designs. Furthermore, obtaining a CapEx cost-effective design using span-protecting p-cycles required to substitute wavelength for glass switched p-cycles, with the cost of ports on the hypothetical fiber switch estimated to be 10% of the cost of wavelength selective ports on a traditional OXC.
From another perspective, wavelength assignment and wavelength continuity constraints required in the restored state for fully transparent p-cycle network architectures certainly bring more complexity and computational issues in the conventional p-cycle network design problem. To overcome those issues, protecting structures can be configured on a waveband (as opposed to wavelength) basis, with each waveband of wavelengths treated as a single unit. In J. M. Simmons, Optical Network Design and Planning (Springer 2008), Chaps. 1&2, pp. 1-59, some background and comparison of wavelength vs. waveband switching are provided for many different aspects. Subsequent analysis brought, for instance, an interesting and useful observation that waveband-level protection has a great potential for eliminating wavelength continuity constraints in fully transparent p-cycle design problems. This contribution pertains to specific cases where whole fiber optics define the wavebands.
From a chronological perspective, W. D. Grover, Mesh-Based Survivable Networks: Options and Strategies for Optical, MPLS, SONET, and ATM Networking (PTR Prentice Hall 2003) was the first to state that p-cycle structures can be configured on a waveband (as opposed to wavelength) basis, with each waveband of wavelengths treated as a single unit. The specific case where whole fiber optics define wavebands is now referred to as whole fiber (or simply, “glass”) switched p-cycles. The concept of glass switched p-cycles is exciting in that to protect against fiber failures or span cuts, wavelength assignment within the failed fibers is irrelevant as long as p-cycle fibers support the same waveband. This means despite the general recognition that requiring wavelength continuity greatly complicates the basic service routing problem, there are no further complications due to protection considerations if p-cycles are used at the fiberswitching level to protect fully transparent transport networks. Implicitly, every wavelength assignment actually retains continuity under protection rerouting because the corresponding wavelength is by definition free for use on the fiber dedicated for protection (if not already in use protecting another failure).
Applying the concept of p-cycles at a fiber level of protection is exciting in that to protect against fiber failures or span cuts, wavelength assignment within the failed fibers is irrelevant as long as p-cycle fibers support the same waveband. This means despite the general recognition that requiring wavelength continuity greatly complicates the basic service routing problem, there are no further complications due to protection considerations if p-cycles are used at the fiber-switching level to protect fully transparent transport networks. Implicitly every wavelength assignment actually retains continuity under protection rerouting because the corresponding wavelength is by definition free for use on the fiber dedicated for protection, if not already in use protecting another failure.
Possibilities for fiber-level protection with p-cycles were first and briefly stated in W. D. Grover, Mesh-based Survivable Networks: Options and Strategies for Optical, MPLS, SONET and ATM Networking (PTR Prentice-Hall 2003), Chap. 10, pp. 659-748. In providing p-cycle design solutions for homogeneous networks, of exactly two fibers per span with identical number of wavelength channels, authors in A. Sack and W. D. Grover, “Hamiltonian p-Cycles for Fiber-Level Protection in Homogeneous and Semi-Homogeneous Optical Networks,” IEEE Network, Special Issue on Protection, Restoration and Disaster Recovery, vol. 18, no. 2, pp. 49-56, March-April 2004 somehow developed the idea (of whole fiber p-cycles). But the study was limited to the usage of Hamiltonian structures only. Although we recognize that an effective p-cycle network design can still be based on a single Hamiltonian with the attraction of a quite easy calculation, we have previously demonstrated that designs involving on the contrary many complementary cycle structures give rise to much more efficiency from the capacity requirement perspectives.
If W. D. Grover, Mesh-Based Survivable Networks: Options and Strategies for Optical, MPLS, SONET, and ATM Networking (PTR Prentice Hall 2003) opened possibilities for fiber-level protection with p-cycles, the seeds of idea have remained unexplored. Perhaps the main reason why whole fiber switched p-cycles have not been seriously challenged is a widespread idea that whole fiber switching operations are very slow.
After a dozen years of history, p-cycles are a now well established and widely studied span-protecting architecture for network survivability. One of the main reasons why p-cycles have been gaining interest over the last decade is low capacity requirement in the designs, in comparison to that needed when using other network protection methods. Another interesting and attractive property of p-cycles is their inherent and efficient response to such advanced questions as single and dual span failure protection, node failure recovery, optical reach control, wavelength assignment and same wavelength protection. But despite so many advantages, the practicability of p-cycles vis-à-vis that of path-protecting pre-cross-connected schemes has remained questionable from the capital expenditure (CapEx) cost perspective because of wavelength switching operations.
In fact, p-cycle configuration types available within the literature typically assume a protection switching based on the wavelength granularity level. Accordingly, authors in D. A. Schupke, M. C. Sheffel, W. D. Grover, Configuration of p-Cycles in WDM Networks with Partial Wavelength Conversion, Photonic Network Communications, Kluwer Academic 6 (3) (2003) 239-252 distinguish between three types of p-cycle configuration in the WDM layer: i.e., opaque, hybrid and fully transparent p-cycle designs.
Although unintentional, authors in A. Sack, W. D. Grover, Hamiltonian p-Cycles for Fiber-level Protection in Homogeneous and Semi-Homogeneous Optical Networks, IEEE Network, Special Issue on Protection, Restoration and Disaster Recovery 18 (2) (2004) p. 49-56 generated the first whole fiber switched p-cycle designs while providing p-cycle network solutions for homogeneous networks, which comprise spans of exactly two fiber optics with identical number of wavelength channels. As a result, only Hamiltonian cycle structures were involved in the solutions. Even though an effective p-cycle network design can still be based on a single Hamiltonian, with the attraction of a quite easy calculation, the preliminary study in D. P. Onguetou, W. D. Grover, p-Cycle Network Design: from Fewest in Number to Smallest in Size, in: Proceedings of the 6th International Workshop on the Design of Reliable Communication Networks (DRCN), La Rochelle, France, 2007 previously demonstrated that designs involving many complementary cycle structures give rise to increased efficiency from spare capacity requirement perspectives.
Overall, configuring p-cycles on a per-wavelength basis is generally more than attractive from the perspectives of flexibility in the routing of working paths and freedom in the selection of protecting structures. But in A. Grue, W. D. Grover, M. Clouqueur, D. A. Schupke, D. Baloukov, D. P. Onguetou, B. Forst, Capex Costs of Lightly Loaded Restorable Networks Under a Consistent WDM Layer Cost Model, in: Proceedings of the IEEE International Conference on Communications (ICC), Dresden, Germany, 2009, we brought to attention that wavelength switching operations in the restored network state greatly increase equipment prices. Where wavelength conversion is allowed, either at p-cycle entry nodes or at every node crossed en route, high equipment costs are incurred because of optic-electronic-optical (o-e-o) conversion from one span to the next, or from the working path to the protection cycle and vice versa.